In the production processes of iron and steel, waste materials are formed that contain oxidized iron and frequently other oxidized metals. These are usually materials in the form of dust in the gas waste streams. This waste is difficult to process because the dust usually has a fine particle size, and merely reintroducing it into a reduction furnace to recover the iron will usually result in it becoming again a part of the waste gas stream. Accordingly, these fine particle size materials, even though they contain a substantial metal content, are essentially worthless.
Them are methods of storing and stabilizing the dust into piles near the steel making facility, but this option is becoming increasingly unacceptable as environmental regulations become stricter and as land values increase. The dust can also be recycled and stabilized into ceramic or building materials. But these methods do not exploit the value of the residual iron and other metals in the waste.
A waste material of common concern is dust from electric arc furnaces, commonly referred to as EAF dust. Electric arc furnaces melt scrap metal through the use of high voltage electrical current. The scrap metal may come from a variety of sources, including; discarded railroad rail, cut sheet steel, discarded structural steel, and scrap automobiles. The scrap metal is added to the electric arc furnaces without separating non-ferrous metals, such as lead, zinc, and cadmium. During the operation of the electric arc furnace these non ferrous metals are vaporized from the scrap, condense into a dust from the waste gas stream and are deposited in the bag house. In addition to these metals, the waste gas stream deposits a large amount of recoverable iron in the bag house. Accordingly, the iron and heavy metal, usually in an oxidized form, are combined in an amorphous EAF dust with particle sizes commonly less than 20 microns. Such EAF dust is now classified as hazardous waste by the EPA due to the lead and cadmium content. As such, extensive procedures must be maintained in order to protect the environment from heavy metal contamination and meet EPA regulations. All the metals in the EAF dust have value and can be reclaimed if an efficient means of separation and reduction of the component dust can be achieved. Additionally, the EAF dust can be rendered non-toxic if the trace heavy metals can be removed from the dust.
Several process have been applied to this problem with differing degrees of success. While these processes have been successful in removing heavy metals, they have been inadequate in recovering the iron, and generally leave a fine iron oxide containing dust of no value.
The most common approach is called fuming. This process utilizes the differing boiling points of the heavy metals to obtain their separation. The dust is heated to temperatures above the boiling points of the metals being separated, causing the metals to evaporate. The evaporated metals are removed as a dust from the gas and condensed in a collection device for further processing. The boiling points of these trace metals are considerably lower than that of iron, which is the largest single component of the dust. After the lead, zinc, and cadmium are separated, the remaining dust consists primarily of iron in the form of iron oxide. Being in a dust form, this material cannot be successfully processed into iron and is left as a waste. Another problem with fuming, is that it is energy intensive, and it also produces a significant amount of its own waste dust.
Another process used to treat EAF dust is electrowinning. This process combines a leaching and precipitation operation with an electrolytic deposition. The EAF dust is first dissolved in an electrolyte to solubilize the lead, zinc, and cadmium. The solution is filtered and then precipitated with a zinc powder to capture the lead and cadmium. The resulting zinc solution is then passed through an electrochemical recovery cell to recover the zinc. This process recovers zinc quite well, but the leaching process does not dissolve the iron oxides and zinc ferrite, which remain as waste materials that must be dried. The dried material, once again, is in the form of a fine dust with little or no value.
EAF dust has also been processed by blending with silicate materials, such as silica sand, clay or cullet, and heated in a furnace to form a vitrified ceramic product. The ceramic is useful as an abrasive, and the EAF dust is rendered nonhazardous, but the valuable metals contained in the dust are not recovered. These metals have been through an expensive refining process just to be converted into a relatively low value material in order to render them nonhazardous.